[BUGFIX] Resolves the errors, which appers if the value is an empty array

Classes/Service/FileBaseXLFService.php

@@ -120,6 +120,10 @@ class FileBaseXLFService extends FileBaseService {
 			} else {
 				$flatData[$constant] = $data['source'];
 			}
+
+			if (empty($flatData[$constant])) {
+				$flatData[$constant] = '';
+			}
 		}
 
 		return $flatData;